The invention relates to polarographic measurement of oxygen concentration, performed using a polarography electrode and a reference electrode.
If oxygen is physically or chemically bound to a substance, there sets in an equilibrium as between free and bound oxygen. Polarographic oxygen-concentration measurements are most easily and accurately performed when this equilibrium is in existence. If this equilibrium becomes disturbed, it thereafter tends to become reestablished, by means of diffusion. For example, if the partial pressure of oxygen is to be ascertained by polarographic determination of free oxygen, then the oxygen which is consumed by the polarographic measuring procedure itself must be replenished, e.g. continually replenished, by means of diffusion. However, due to the low speed at which the replenishment by diffusion proceeds, the replenishment is not complete. As a result, a concentration gradient develops within the medium being analyzed, and there is simulated a value of the oxygen partial pressure which is lower than the actual value of the partial pressure.
A variety of techniques are known for overcoming this difficulty. One technique involves setting the medium to be analyzed into convective motion during the course of the polarographic measurement; this speeds the rate at which oxygen consumed by the measurement process itself can be replenished from portions of the medium not most directly involved in the electrolytic current path. Another known technique involves the use of electrodes whose effective surfaces are extremely small; this makes the electrolytic current path through the medium being analyzed so "thin" that oxygen consumed within this zone is very quickly replenished by diffusion from the medium surrounding this "thin" depletion zone, so quickly that equilibrium is maintained substantially uninterruptedly with substantially no development of the aforementioned concentration gradient. Finally, because the correlation between the shape and characteristics of these diffusion fields, on the one hand, and the shape and effective surface area of the electrodes used, on the other hand, is well understood in the art, it is also known to take this correlation expressly into account for the particular electrodes employed, and to correct the too low value of oxygen partial pressure simulated during the polarographic measurement, by introducing corresponding correction factors (GRUNEWALD, Diss. Marburg 1966).
However, these prior-art techniques are not applicable, when one is faced with the still further complications which arise when the oxygen whose concentration is to be measured is carried within media undergoing highly fluctuating perfusion, for example the oxygen in tissues through which blood is perfused.
In such cases, i.e., when the measuring electrodes are introduced into the capillary system, even empirically and/or theoretically devised models such as referred to above are inadequate for the development of correction factors, and so the actual magnitude of the oxygen partial pressure in the tissue cannot be ascertained with any directness.
Inasmuch as the case described above is very important, i.e., is the typical example of polarography applied to physiological measurements, certain techniques have been devised to permit oxygen concentration measurement, even in these circumstances. It is known to simultaneously determine the perfusion and the oxygen partial pressure, and then based on that information to calculate the perfusion efficiency. Then, if the rate at which oxygen is consumed by the polarographic apparatus is kept small, i.e., by maintaining the effective surface area of the electrodes small in the way described above, and if the perfusion is measured for example using the heat-flow procedure of HENSEL, it becomes possible to reliably ascertain the value of oxygen partial pressure, e.g., transcutaneously, and this value will correspond to the local arterial oxygen partial pressure in the capillary system (see German allowed patent application DT-AS No. 22 55 879or corresponding U.S. Pat. No. 3,918,434).
However, a disadvantage of the aforementioned technique is that the electrodes employed must have an effective surface area of only a few square microns, to keep the oxygen consumption low. Electrodes having so minute an effective surface area are not dimensionally reliable except for relatively short periods of use. On the other hand, if electrodes are used having an effective surface area of more than 100 square microns, then the long-term dimensional reliability of the electrodes is certainly satisfactory; but then the aforedescribed errors attributable to the slowness at which oxygen is replenished by diffusion processes, are again no longer tolerable. An additional disadvantage of this known technique is that the consolidation of a polarographic probe and a thermo probe into a single component makes for a component which is very complex mechanically and highly malfunction-prone.
Still another way of dealing with the difficulty presented by the sluggishness of diffusion processes resides in the use of pulse techniques (Pflugers Arch. 276/pp. 415 ff.). In these techniques, the electrodes employed can be of comparatively large effective surface area. However, the oxygen partial pressure in the tissue of interest is not detected directly. Instead, what is measured is the oxygen partial pressure in a special chamber arranged intermediate the tissue of interest and the measuring electrodes. The oxygen partial pressure within this special chamber must be maintained in equilibrium with that in the tissue itself. The contents of the measuring chamber are exhausted during the course of the polarographic measurement. To generate an accurate measurement with this apparatus, it is critical that the dimensions of the measuring chamber be very precisely correlated with the effective surface area of the electrodes and with the length of the time interval during which the polarographic measurement is performed. The technical complexities of this method are very considerable, and it has not achieved widespread use.
Finally, it is also known to measure perfusion using the hydrogen-clearance technique (Pflugers Arch. 348/225). With this technique, a pair of electrodes is first used to generate hydrogen within the perfused medium to be analyzed. The generated hydrogen is then washed out by the perfusion flow itself. If one polarographically measures the hydrogen partial pressure, then by knowing the drop in the hydrogen partial pressure attributable to this wash-out, information can be derived concerning the actual perfusion rate.